1. Field of the Invention
The invention relates to an magnetic brake and more particularly to an improved electromechanical braking device especially useful for motor vehicles.
2. Description of the Prior Art
One such magnetic brake is known from U.S. Pat. No. 5,185,542. The known magnetic brake has one rotatable part and one rotationally fixed part, which are in frictional or positive engagement with one another in a braking position of the magnetic brake, so that the rotatable part is held or at least braked by the rotationally fixed part, and which in a released position of the magnetic brake are free of one another, so that the rotatable part is freely rotatable. For actuation, the known magnetic brake has a spring element, which presses the rotationally fixed part or the rotatable part against the respectively other part, as well as an electromagnet, which by being supplied with current disconnects the rotatable part and the rotationally fixed part from one another counter to the force of a spring element; that is, the spring element puts the magnetic brake in its braking position and keeps it there, and the magnetic brake can be released by means of the electromagnet. It is equally possible to put a magnetic brake into the braking position by supplying current to the electromagnet, while conversely a spring element releases the magnetic brake.
The magnetic brake has the disadvantage that in the event of a defect, or in other words if its electromagnet or its power supply fails, it cannot be actuated.
The magnetic brake of the invention as defined by the characteristics of claim 1 has a second electromagnet, with which it is actuatable. The magnetic brake of this invention is advantageous because it has a magnetic brake that is actuatable selectively by its first or second electromagnet; the two electromagnets are redundant. This has the advantage of high operational reliability of the magnetic brake of the invention; failure is virtually precluded.
Preferably, the two electromagnets are each connected to their own, mutually independent power supplies, so that there is also redundance in terms of the power supply for actuating the magnetic brake, which further reduces the likelihood of failure of the magnetic brake (claim 2).
In a preferred feature of the electromechanical wheel braking device, the magnetic brake is embodied in bistable form (claim 3); that is, it remains both in the released position and in the braking position without current being supplied to its electromagnets. The electromagnets serve to switch the magnetic brake over from the released position to the braking position and conversely from the braking position into the released position. For the switchover between the two positions, only a brief supply of current selectively to the first or the second electromagnet is necessary. The bistable embodiment of the magnetic brake can be done for instance with the aid of a permanent magnet, which keeps the magnetic brake in one of its two positions counter to the force of a spring element, while conversely, after the switchover by means of one of its two electromagnets, the magnetic brake is kept in the other position counter to the force of the permanent magnet by the spring element; the force of the permanent magnet in this other position of the magnetic brake is weakened by an air gap, caused by the switchover, in its magnetic circuit.
In a feature of the invention in accordance with claim 4, the magnetic brake is part of an electromechanical braking device for a motor vehicle; it serves to lock the electromechanical braking device in the actuated position, so that a braking force generated with the braking device is kept constant, without current being supplied to the electromechanical braking device. Supplying current to the electromechanical braking device is necessary solely to generate or boost the braking force and/or to reduce the braking force, which is understood also to mean a complete release of the electromechanical braking device. The electromechanical braking device can as a result be used as a parking brake, which once a braking force has been brought to bear maintains it without current being supplied. The electromechanical braking device can also be locked during a braking event with constant braking force using the magnetic brake, so that the braking force is maintained without current being supplied to the braking device. Only in order to vary the braking force is the magnetic brake switched into its released position and is current supplied to the braking device in such a way that its braking force varies in the desired way. In a preferred feature, the electromechanical braking device is embodied in non-self-locking fashion; that is, it releases itself when there is no current to the electric motor and the magnetic brake is released, because of a reaction force to the contact pressure force with which its friction brake linings are pressed against a brake body, such a brake disk or a brake drum, except for a negligible residual braking force. This feature of the invention has the advantage that the electromechanical braking device can be released in every case, because of the redundance of the magnetic brake, even if its electric motor or its power supply fails. It is therefore unnecessary to provide a second electric motor to release the electromechanical braking device in the event of a defect. The expense for enabling the release of the electromechanical braking device even in the event of a defect is minimal; it is limited to the provision of a second electromagnet for the magnetic brake.